As no other Japanese replies, I will answer your questions. Though I believe most of Japanese would agree with me, there can be some Japanese denys me.

a) In General, no Japanese will use "MAGO-MUSUME" in verbal(not written) communication. Even if the grandson/daughter is over thirty, the gender would not be in question.
b) One will not call one's younger relatives with "-SAN" unless the relative is "in law".
c) Japanese will never use "IDAINA:great" to call relatives.

How do you learn these examples ? There are too much misunderstandings for me to correct.